Can I do an engineering degree as part of my job if I already did a degree in a different subject in the past?

The quick answer is yes however it depends what you join (I assume the REME if you wanted to do a Mech Eng degree). Both officers and soldiers (although it tends to be a Snr NCO level) can complete a degree sometimes it will be part of a course or requirement (in which case the army will fund the entire degree) or it may be something you choose to do - in which case the army will contribute towards it. The Army is all about life long development and therefore is very supportive to those doing further education - whether it's with funding, time off to complete it or both. I hope this helps.